Difference between revisions of "Immobilization of salts"

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<bibimport />
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Author: [[user:Hschwarz|Hans-Jürgen Schwarz]]<br>
 
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English Translation by [[user:SLeithaeuser|Sandra Leithäuser]]<br>
Author: [[user:Hschwarz|Hans-Jürgen Schwarz]]
 
 
<br>
 
<br>
 
back to [[Salt conversion]]
 
back to [[Salt conversion]]
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==Barium method==
 
==Barium method==
  
The barium method bases on the theory, that in the presence of readily soluble sulfate salts, the sulfate precipitates with a readily soluble barium compound to become barium sulfate, and hereby removes the damaging salt from the system.
+
The so-called "barium" method is based on the fact that when a soluble barium compound is added to a solution containing soluble sulfate salts, the highly insoluble barium sulfate will precipitate out and be 'immobilized' given its extremely low solubility. However, other soluble salts may remain from the counter ions of the original and barium compound added. These, however, may be less damaging.
  
  
 
=== Gypsum conversion  ===
 
=== Gypsum conversion  ===
  
The conversion of gypsum <ref>http://www.fead-gmbh.de/Naturstein%20Gips.html, gesehen 17.1.2011</ref>, which particularily refers to the removal of gypsum efflorescence and gypsum crusts has been in use for several years <bib id="Hammer:1996" /> and has proven to be effective, when used in appropriate cases.  
+
The above approach is used for the conversion of gypsum <ref>http://www.fead-gmbh.de/Naturstein%20Gips.html, gesehen 17.1.2011</ref>, in particular for gypsum efflorescence and crusts, into barium sulfate. It has been in use for several years <bib id="Hammer:1996" /> and has proven to be effective when used appropriately.  
Gypsum conversion <ref> http://www.baufachinformation.de/denkmalpflege.jsp?md=1988017124771, gesehen 17.1.2011</ref> is carried out in five steps <bib id="Matteini:1991" />:
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The gypsum conversion <ref> http://www.baufachinformation.de/denkmalpflege.jsp?md=1988017124771, gesehen 17.1.2011</ref> is best carried out in five steps <bib id="Matteini:1991" />:
  
 
'''1. Dissolution of gypsum'''  
 
'''1. Dissolution of gypsum'''  
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CaSO<sub><font size="1">4</font></sub>•2H<sub><font size="1">2</font></sub>O + (NH<sub><font size="1">4</font></sub>)<sub><font size="1">2</font></sub>CO<sub><font size="1">3</font></sub> → (NH<sub><font size="1">4</font></sub>)<sub><font size="1">2</font></sub>SO<sub><font size="1">4</font> </sub>+ CaCO<sub><font size="1">3</font></sub> + 2H<sub><font size="1">2</font></sub>O  
 
CaSO<sub><font size="1">4</font></sub>•2H<sub><font size="1">2</font></sub>O + (NH<sub><font size="1">4</font></sub>)<sub><font size="1">2</font></sub>CO<sub><font size="1">3</font></sub> → (NH<sub><font size="1">4</font></sub>)<sub><font size="1">2</font></sub>SO<sub><font size="1">4</font> </sub>+ CaCO<sub><font size="1">3</font></sub> + 2H<sub><font size="1">2</font></sub>O  
  
In the first step the application of an ammonium carbonate poultice leads to the conversion of gypsum into soluble ammonium sulfate. It migrates to one part into the poultice and to another part it stays in the the surface layer and eventually migrates into lower layers. If calcite forms under the surface of the plaster, it achieves a positive, consolidating effect; if it forms on the surface, it has to be removed dilligently. Excess ammonium carbonate decomposes to become ammonia, carbon dioxide and water. (Ammonium carbonate alters proteinaceous coatings).
+
The first step is the application of an ammonium carbonate containing poultice to the surface to be treated. This leads to the conversion of the relatively insoluble gypsum into soluble ammonium sulfate that partly migrates into the poultice. The remaining ammonium sulfate will eventually migrate and distribute itself in the material. The reaction will also form calcium carbonate (calcite) and this will have a consolidating effect. However, any calcite forming on the surface has to be removed diligently to avoid its whitening effect. Any excess ammonium carbonate decomposes into gaseous ammonia, carbon dioxide and water. It is to be taken into account that ammonium carbonate may alter decorative mural paintings that contain proteinaceous additives.
  
  
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(NH<sub><font size="1">4</font></sub>)<sub><font size="1">2</font></sub>SO<sub><font size="1">4</font></sub> + Ba(OH)<sub><font size="1">2</font></sub> → BaSO<sub><font size="1">4</font></sub>↓ + 2NH<sub><font size="1">3</font></sub><font size="1"></font>+ 2H<sub><font size="1">2</font></sub>O  
 
(NH<sub><font size="1">4</font></sub>)<sub><font size="1">2</font></sub>SO<sub><font size="1">4</font></sub> + Ba(OH)<sub><font size="1">2</font></sub> → BaSO<sub><font size="1">4</font></sub>↓ + 2NH<sub><font size="1">3</font></sub><font size="1"></font>+ 2H<sub><font size="1">2</font></sub>O  
  
The insoluble ammonium sulfate of the first reaction becomes the insoluble barium sulfate.
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The second step is the application of a barium hydroxide containing poultice. The soluble ammonium sulfate in the substrate will react to form the highly insoluble barium sulfate. The counter ions, hydroxyl and ammonium will evaporate as ammonia and water.
  
  
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Ba(OH)<sub><font size="1">2</font></sub> + CO<sub><font size="1">2</font></sub><font size="1"></font>→ BaCO<sub><font size="1">3</font></sub>↓ + H<font size="1">2</font>O  
 
Ba(OH)<sub><font size="1">2</font></sub> + CO<sub><font size="1">2</font></sub><font size="1"></font>→ BaCO<sub><font size="1">3</font></sub>↓ + H<font size="1">2</font>O  
  
Excess barium hydroxide with ambient carbon dioxide converts to barium carbonate. This has a consolidating effect.
+
Any excess barium hydroxide will react with ambient carbon dioxide to form barium carbonate. This has a consolidating effect.
  
  
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Ba(OH)<sub><font size="1">2</font></sub> + CaCO<sub><font size="1">3</font> </sub>→ BaCO<sub><font size="1">3</font></sub>↓+ Ca(OH)<sub><font size="1">2</font></sub>  
 
Ba(OH)<sub><font size="1">2</font></sub> + CaCO<sub><font size="1">3</font> </sub>→ BaCO<sub><font size="1">3</font></sub>↓+ Ca(OH)<sub><font size="1">2</font></sub>  
  
A heterogeneous reaction converts the outer regions of the calcite corns into calcium hydroxide gel.
+
There may be an exchange reaction between any existing calcite in the substrate and the barium hydroxide where the outer surface of the calcite grains turn into calcium hydroxide gel while barium carbonate may also accrue on other crystals.
  
  
 
'''5.''' Ca(OH)<sub><font size="1">2</font></sub> + CO<sub><font size="1">2</font></sub> → CaCO<sub><font size="1">3</font></sub>↓+ H<sub><font size="1">2</font></sub>O  
 
'''5.''' Ca(OH)<sub><font size="1">2</font></sub> + CO<sub><font size="1">2</font></sub> → CaCO<sub><font size="1">3</font></sub>↓+ H<sub><font size="1">2</font></sub>O  
  
A consolidating effect is achieved due to carbonation. (The reactions 4 and 5 have not yet been investigated well enough and need better understanding.)
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Any calcium hydroxide formed will re-carbonate increasing the consolidating effect. The reactions 4 and 5 have not as yet been thoroughly investigated and require further studies.  
  
The method should not be used when ''nitrate'' is present in high concentration, when ''organic binders'' are present and when an ''adhesive effect'' is required.
+
IMPORTANT NOTE:
 +
The method should not be used in the presence of a high concentration of ''nitrates'' or ''organic binders''.
 +
This approach does not provide an ''adhesive effect''.
  
Nitrates cause the formation of barium nitrate, which is slightly soluble and leads to visible crystallization on the surface. The organic binders in tempera or oil paintings do not tolerate the high alkalinity of barium hydroxide and lead to hydrolysis or saponification. '''Matteini''' <bib id="Matteini:1991" /> is of the opinion, that in old paintings, these organic binders have largely transformed to become inorganic compounds (like calcium oxalate) and the above reactions will not necessarily take place, the use of this method can therefore be justified.   
+
Nitrates will result in the formation of barium nitrate, which is slightly soluble leading to its visible crystallization on the surface. The organic binders in tempera or oil paintings do not tolerate the high alkalinity of barium hydroxide resulting in their hydrolysis or saponification. '''Matteini''' <bib id="Matteini:1991" /> is of the opinion, that in old paintings, these organic binders have largely transformed into inorganic compounds such as calcium oxalate and the above reactions will not necessarily take place, the use of this method can therefore be justified.   
  
 
=== [[Magnesium sulfate conversion]]===
 
=== [[Magnesium sulfate conversion]]===
  
Magnesium sulfate, like gypsum, can also be converted into the slightly soluble barium sulfate and ideally become magnesium carbonate, rendering harmless the damaging salt<bib id="Friese.etal:1999"/>.
+
Similarly, magnesium sulfate can also be converted into the highly insoluble barium sulfate and ideally turn into the relative insoluble magnesium carbonate, rendering harmless the damaging salt <bib id="Friese.etal:1999"/>.
 
 
 
 
  
 
== Treatment with lead hexafluorosilicate ==
 
== Treatment with lead hexafluorosilicate ==
  
A chemical salt conversion using lead hexafluorosilicate was sometimes recommended for the treatment with hydrophobic restoration plasters/mortars (link), because the restoration mortars are not hydrophobic, when they are freshly applied. Salts can therefore easily migrate into the pore structure of the plaster. To reduce the risk of a rapid salinization of the restoration mortar, the pretreatment of the substrate with a "salt treatment agent" is recommended. The different manufacturers suggest a variety of combinations. The most important ones are produced on the basis of lead hexafluorosilicate. Barium containing agents are also on offer. Even though a certain effectiveness of the treatment has been established, it must be stressed that soluble lead salts are a risk to the workforce and to the environment. For this reason, attempts should be made to achieve the same effect (the prevention of a rapid salt migration into the new plaster) through the use of impregnating agents, that have a hydrophobic and a compacting effect on the capillaries.  Once the agents are activated and capillary compacting and hydrophobizing takes place, a reduction of salt migration to the surface is effected. Potential problems when applying the plaster are to be considered. As active ingredients sodium silicate compounds and potassium methyl siliconate/ fluate, i.e. salts of  hexafluorosilicic acid are usually present.  
+
In the past, another conversion method based on the use of lead hexafluorosilicate was sometimes recommended when commercial restoration [[Renders/Mortars]], called Sanierputze in German, to prevent salts from migrating into the fresh render. The commercial restoration renders are formulated with a water repellent, mostly Na-oleate (that with the render turns into Ca-oleate) or Zn-stereate. Hydrophobicity of the render will prevent salts from penetrating it, and therefore, it will have a longer service-life. To be taken into account it that these commercial restoration renders were formulated mostly for old farm houses where walls tend to be saturated with salts. The render is applied to both sides so that salts are "trapped" within the wall which will remain moist due to salt hygrocopicity, but the renders will be dry because of their hydrophobicity. Different manufacturers suggest a variety of combinations. The most important ones are produced on the basis of lead hexafluorosilicate. Barium containing agents are also offered.  
  
 +
Even though a certain effectiveness of the treatment has been established, it must be stressed that soluble lead salts are a risk to the workforce and to the environment and should be avoided. For this reason, attempts should be made to achieve the same effect (the prevention of a rapid salt migration into the new plaster) through the use of additives to the render mix to reduce pore sizes and turn it hydrophobic. For this purpose sodium silicate compounds and potassium methyl siliconate/fluate, i.e., salts of hexafluorosilicic acid are usually used.
 +
This will reduce salt migration to the surface of the set render. However, there may be some problems when applying the render. As active ingredients
 
Lead hexafluorosilicate reacts with sulfate and chloride compounds in complex reactions, forming a variety of products, which are nearly all hardly soluble or insoluble.
 
Lead hexafluorosilicate reacts with sulfate and chloride compounds in complex reactions, forming a variety of products, which are nearly all hardly soluble or insoluble.
  
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== Literatur ==
 
== Literatur ==
  
<bibprint/>
+
<biblist/>
  
  
[[category: Measures]][[category: Hschwarz]][[category: inProgress]]
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[[category: Measures]][[category: Schwarz,Hans-Jürgen]][[category: approved]]

Latest revision as of 20:42, 11 November 2017

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser

back to Salt conversion


Barium method

The so-called "barium" method is based on the fact that when a soluble barium compound is added to a solution containing soluble sulfate salts, the highly insoluble barium sulfate will precipitate out and be 'immobilized' given its extremely low solubility. However, other soluble salts may remain from the counter ions of the original and barium compound added. These, however, may be less damaging.


Gypsum conversion

The above approach is used for the conversion of gypsum [1], in particular for gypsum efflorescence and crusts, into barium sulfate. It has been in use for several years [Hammer:1996]Title: Salze und Salzbehandlung in der Konservierung von Wandmalerei und Architekturoberfläche.
Author: Hammer, Ivo
Link to Google Scholar
and has proven to be effective when used appropriately. The gypsum conversion [2] is best carried out in five steps [Matteini:1991]Title: In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments
Author: Matteini, Mauro
Link to Google Scholar
:

1. Dissolution of gypsum

CaSO4•2H2O + (NH4)2CO3 → (NH4)2SO4 + CaCO3 + 2H2O

The first step is the application of an ammonium carbonate containing poultice to the surface to be treated. This leads to the conversion of the relatively insoluble gypsum into soluble ammonium sulfate that partly migrates into the poultice. The remaining ammonium sulfate will eventually migrate and distribute itself in the material. The reaction will also form calcium carbonate (calcite) and this will have a consolidating effect. However, any calcite forming on the surface has to be removed diligently to avoid its whitening effect. Any excess ammonium carbonate decomposes into gaseous ammonia, carbon dioxide and water. It is to be taken into account that ammonium carbonate may alter decorative mural paintings that contain proteinaceous additives.


2. Precipitation of the insoluble salts

(NH4)2SO4 + Ba(OH)2 → BaSO4↓ + 2NH3+ 2H2O

The second step is the application of a barium hydroxide containing poultice. The soluble ammonium sulfate in the substrate will react to form the highly insoluble barium sulfate. The counter ions, hydroxyl and ammonium will evaporate as ammonia and water.


3. First consolidating reaction

Ba(OH)2 + CO2→ BaCO3↓ + H2O

Any excess barium hydroxide will react with ambient carbon dioxide to form barium carbonate. This has a consolidating effect.


4. The second consolidating reaction

Ba(OH)2 + CaCO3 → BaCO3↓+ Ca(OH)2

There may be an exchange reaction between any existing calcite in the substrate and the barium hydroxide where the outer surface of the calcite grains turn into calcium hydroxide gel while barium carbonate may also accrue on other crystals.


5. Ca(OH)2 + CO2 → CaCO3↓+ H2O

Any calcium hydroxide formed will re-carbonate increasing the consolidating effect. The reactions 4 and 5 have not as yet been thoroughly investigated and require further studies.

IMPORTANT NOTE: The method should not be used in the presence of a high concentration of nitrates or organic binders. This approach does not provide an adhesive effect.

Nitrates will result in the formation of barium nitrate, which is slightly soluble leading to its visible crystallization on the surface. The organic binders in tempera or oil paintings do not tolerate the high alkalinity of barium hydroxide resulting in their hydrolysis or saponification. Matteini [Matteini:1991]Title: In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments
Author: Matteini, Mauro
Link to Google Scholar
is of the opinion, that in old paintings, these organic binders have largely transformed into inorganic compounds such as calcium oxalate and the above reactions will not necessarily take place, the use of this method can therefore be justified.

Magnesium sulfate conversion

Similarly, magnesium sulfate can also be converted into the highly insoluble barium sulfate and ideally turn into the relative insoluble magnesium carbonate, rendering harmless the damaging salt [Friese.etal:1999]Title: Salze im Mauerwerk - Möglichkeiten zur Entsalzung und zur Salzumwandlung
Author: Friese, Peter; Protz, A.
Link to Google Scholar
.

Treatment with lead hexafluorosilicate

In the past, another conversion method based on the use of lead hexafluorosilicate was sometimes recommended when commercial restoration Renders/Mortars, called Sanierputze in German, to prevent salts from migrating into the fresh render. The commercial restoration renders are formulated with a water repellent, mostly Na-oleate (that with the render turns into Ca-oleate) or Zn-stereate. Hydrophobicity of the render will prevent salts from penetrating it, and therefore, it will have a longer service-life. To be taken into account it that these commercial restoration renders were formulated mostly for old farm houses where walls tend to be saturated with salts. The render is applied to both sides so that salts are "trapped" within the wall which will remain moist due to salt hygrocopicity, but the renders will be dry because of their hydrophobicity. Different manufacturers suggest a variety of combinations. The most important ones are produced on the basis of lead hexafluorosilicate. Barium containing agents are also offered.

Even though a certain effectiveness of the treatment has been established, it must be stressed that soluble lead salts are a risk to the workforce and to the environment and should be avoided. For this reason, attempts should be made to achieve the same effect (the prevention of a rapid salt migration into the new plaster) through the use of additives to the render mix to reduce pore sizes and turn it hydrophobic. For this purpose sodium silicate compounds and potassium methyl siliconate/fluate, i.e., salts of hexafluorosilicic acid are usually used. This will reduce salt migration to the surface of the set render. However, there may be some problems when applying the render. As active ingredients Lead hexafluorosilicate reacts with sulfate and chloride compounds in complex reactions, forming a variety of products, which are nearly all hardly soluble or insoluble.

The reaction may be as follows:

Na2SO4 (s) + PbSiF6(s) → PbSO4 (sls)+ Na2SiF6 (sls)

Na2CO3 (s) + PbSiF6 (s) → PbCO3(is) + Na2SiF6 (sls)

MgSO4 (s) + PbSiF6 (s) → PbSO4 (sls) + MgSiF6 (sls)

2NaCl (s) + PbSiF6 (s) → PbCl2 (sls) + Na2SiF6 (sls)

(sls - slightly soluble; s - soluble; is - insoluble)

Weblinks

Literatur

[Friese.etal:1999]Friese, Peter; Protz, A. (1999): Salze im Mauerwerk - Möglichkeiten zur Entsalzung und zur Salzumwandlung. In: Venzmer, H. (eds.): Entfeuchtung/Entsalzung 10. Hanseatische Sanierungstage FAS - Schriftenreihe Heft 10, 211-230Link to Google Scholar
[Hammer:1996]Hammer, Ivo (1996): Salze und Salzbehandlung in der Konservierung von Wandmalerei und Architekturoberfläche.. In: Pursche, Jürgen (eds.): Salzschäden an Wandmalereien, Bayerisches Landesamt für Denkmalpflege, 81-106Link to Google Scholar
[Matteini:1991]Matteini, Mauro (1991): In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments. In: Cather, Sharon (eds.): The Conservation of Wall Paintings: Proceedings of a symposium organized by the Coutrauld Institut of Art and the Getty Conservation Institute, London, July 13-16, 1987, The Getty Conservation Institute, 137-148Link to Google Scholar